1. Technical Field
The present invention relates to a filter, more particularly to a cavity filter that includes resonators.
2. Description of the Related Art
With advances in mobile communication, there have been rapid increases in demand for RF equipment such as filters, duplexers, multiplexers, and the like. RF equipment may be used in the filtering, separation, and transfer of signals in places such as base stations, etc., in a mobile communication system.
An RF filter is a device for passing the signals of a particular frequency band. In devices that require high power, such as in the base station of a mobile communication system, a cavity filter having a cavity-based structure is mainly used.
A cavity filter, the structure of which may include multiple cavities formed within the filter with resonators installed inside the cavities, is a filter that performs the filtering by way of resonance in each of the cavities.
One of the most frequently used resonators in a cavity filter is the coaxial resonator, which is structured to have a cylindrical form with a hole or recess formed therein.
As mobile communication systems require transmission and reception performance of higher and higher sensitivity, there is a critical demand for good voice call quality and data transmission. Thus, the filter installed in a mobile communication system such as a base station is required to provide steep attenuation properties for reliably eliminating spurious emissions while providing a low loss.
A filter having a high Q value to satisfy the above requirements is the TM mode dielectric resonator filter. The TM mode dielectric resonator filter may be electrically connected with the cover or housing of the filter that is in an electrically grounded state to optimize temperature properties.
Recent times have seen active research on cavity filters that are made to have a stable grounding structure between the dielectric resonators and the filter cover or housing. In order for such a cavity filter to provide stable properties that adapt to changes in temperature, etc., a stable joining may be needed between a dielectric resonator and the ground.
The mobile communication system is also required to have smaller equipment. In particular, there has been an increase in the number of low-output compact base stations that control communication for compact cells, and accordingly, there is an increase in demand for smaller sizes in the equipment included in base stations. Thus, there is a continuing demand for smaller size also in the cavity filter using resonators.
In the past, resonators having a step-impedance structure were used, in which the shape of the coaxial resonator was altered to provide a smaller size for the cavity filter. However, it may be difficult to achieve the amount of size reduction as required in a current base station by changing only the shape of the coaxial resonator.